Conventionally, photolithography that uses a polyamic acid precursor of photosensitive polyimide has been mainly used as a method for forming a micropattern of polyimide. The method includes transferring a pattern to a surface of a substrate, which is coated with a photosensitive polyimide precursor, through light exposure using a photomask, performing heat treatment (PEB: post exposure bake) as needed, removing exposed portions or unexposed portions using a developing solution, and performing heat treatment on a pattern of the polyimide precursor obtained through the development, thereby converting the pattern into a pattern of polyimide (see Patent Literature 1).
However, such a method has a problem in that a pattern structure shrinks by 30 to 50% due to a thermal imidization reaction of the amic acid precursor of polyimide, and it is thus difficult to form a microstructure with a sharp line edge or a rectangular cross-section, or a submicrometer pattern.
In contrast, Patent Literature 2 introduces a method for producing ink-state modified polyimide that has been imidized, and a patterning method using photolithography. The patterning method includes (1) a step of applying a photosensitive polyimide composition, which contains modified polyimide, a photosensitizing agent, a thermal curing agent, and a solvent, to a substrate to form a film thereon, (2) a step of heating the obtained film to remove the solvent, (3) a step of exposing the composition with the solvent removed to light through a photomask, (4) a step of performing development after the light exposure, and (5) a step of heating the composition to a temperature of greater than or equal to the curing temperature of the thermal curing agent after the development.
The greatest advantage of the above method is that shrinkage due to an imidization reaction does not occur during the thermal curing step because ink-state polyimide that has been imidized is used. However, as the dissolution properties in a developing solution and in a solvent are not good in the polyimide state, it is difficult to form a micropattern with photolithography. So far, only experimental proofs of patterns with a scale of down to several tens of micrometers have been reported.
As another method for solving the problem of shrinkage due to an imidization reaction, there is also known a method for forming a micropattern through laser processing, etching, or the like, using polyimide that has been imidized.
However, although laser processing, which is a sequential processing method, is suitable for forming a hole, such as a via shape, it has low productivity for forming complex patterns, and further, it is difficult to form submicrometer-scale patterns with laser processing, which is problematic.
In addition, etching involves a long process including, for example, (1) a step of forming a polyimide film on a substrate, (2) a step of forming a resist pattern through photoresist application, pattern exposure through a photomask, PEB, a developing process, and the like, (3) a step of etching the polyimide film using the resist pattern as etching resist, (4) a step of removing the photoresist layer, and (5) processing the pattern of polyimide obtained through the etching. Further, polyimide has problems in that while it has excellent chemical stability, it has quite a low etching rate and thus has low productivity.
Meanwhile, a thermal imprinting technique has been used as a simpler method for forming a polyimide pattern in comparison with the conventional techniques of forming a micropattern of polyimide. The method includes, in order to obtain moldability suitable for imprint molding, heating polyimide to a temperature of greater than or equal to the glass-transition temperature (Tg) until it softens, performing pressure molding by pressing projection/recess patterns of a mold against the polyimide, cooling the polyimide and the mold to a temperature of less than or equal to Tg in such a state, and removing the mold, thereby forming projection/recess patterns on the surface of the polyimide. Using such a thermal imprint molding technique is advantageous in that the number of steps can be significantly reduced as compared to that of the aforementioned photolithography.
However, as the glass-transition temperature (Tg) of polyimide is typically greater than or equal to 300° C., heating should be performed at a temperature as high as 300 to 400° C., which may influence the pattern transfer accuracy due to thermal expansion, decrease the alignment accuracy, or have a problem of thermal stress.
There is still another problem in that a highly heat-resistant mold should be selected at a temperature of greater than or equal to 300° C., or the mold releasability would decrease as a result of a release film, which is formed on the surface of the mold, having been thermally oxidized. Further, there is also a problem in that the heating and cooling process time becomes longer, which can result in increased process cost.
Meanwhile, Patent Literature 3 describes forming a member for a display, which can be cured at a temperature of less than or equal to 250° C. through thermal imprinting, using a curable composition containing polyimide and resin other than polyimide. However, this technique is intended to form partitions for a plasma display with a pitch of about 160 μm and a line width of about 20 μm, and does not consider forming submicrometer patterns.